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cathode is made from a rectangular piece 10x15 cm. with a
long tongue, the square portion being bent into the form of a
cylinder surrounding the porous cell, and the projecting tongue
serving as attachment to the circuit (see Fig. 77, p. 144). It is
advisable to reverse the current before use so as to produce a
metallic surface.
The whole apparatus is placed in a good freezing mixture.
The electrodes are connected in circuit with an ammeter and
resistance as described on p. 144. The reduction requires theoreti-
cally 9 ampere-hours and the strength of current may vary
between moderately wide limits (2—6 amperes) per 100 sq. cm.
of cathode surface. The cathode liquid should be frequently
stirred so as to bring the suspended ^oxalic acid into solution,
and, as the yield of glyoxylic acid depends on efficient cooling,
it is important that the temperature should not exceed 10°. If
the temperature is allowed to rise, glycollic acid is formed.
The glyoxylic acid is separated as the calcium salt. The
cathode liquid is poured into a basin and the sulphuric and
unchanged oxalic acid precipitated with standard baryta solution.
The mixture is filtered and the nitrate is concentrated in vactio
at 60° (see p. 94), neutralised in the cold with calcium
carbonate, boiled up for a short time and filtered. As calcium
glyoxylate is only slightly soluble in cold water (i part in 140
of water at i8c) the greater portion crystallises on cooling. If
calcium glycollate, which is much more soluble, is present, it
may be separated from the filtrate by concentrating the solu-
tion on the water-bath and precipitating with spirit. To obtain
free glyoxylic acid, the calcium salt is dried and suspended
in water, the calculated quantity of oxalic acid added and the
mixture filtered. The filtrate is evaporated in a vacuum
desiccator, when the glyoxylic acid remains as a viscid liquid
which may crystallise on long standing,
Properties.—Crystallises in rhombic prisms ; very soluble in
Reactions,—i. Adda few drops of the acid solution or solu-
tion of the calcium salt to a few c.c. of ammonia-silver nitrate
and warm in hot water. A silver mirror is deposited.
2. To the acid, neutralised with potassium carbonate, or to the